Technical Field
The present disclosure relates generally to optical detection. More particularly, the present disclosure relates to heterodyne detection of images. Specifically, the present disclosure relates to an extended field heterodyne detection apparatus.
Background Information
Optical heterodyne detection of a modulated input signal occurs through mixing the input signal with a stable, fixed frequency signal (often called a local oscillator) in a device such as a tube, transistor, or diode mixer to create an output signal. The combination of the two signals may then produce an output signal that is either equal to either the sum or the difference of the two input frequencies. The output signal may then be filtered, rectified, and/or amplified. The output signal may also be analyzed to determine the frequency, amplitude, or phase of the input signal and thereby yield an image or other useful information based on the input signal. This process may allow the detection of otherwise undetectable high frequency signals, and has a tremendous variety of uses, such as applications in military (thermal imaging, target tracking, surveillance, communications, etc.), atmospheric analysis, and astronomy, just to name a few.
Heterodyne detection may be done passively or actively. In passive detection, the input signal consists of the background radiation derived from a target. The background radiation would then be heterodyned with a local oscillator signal to create the heterodyned signal. In active detection, a reference signal, such as a laser, may be directed toward and reflected off of the target. The signal that is reflected from the target will be modulated by the target. The reflected signal may then be heterodyned with a local oscillator to create the heterodyned signal. In both passive and active detection, properties of the target may be determined via analysis of the heterodyned signal.
One of the objectives of an optical system utilizing heterodyne detection is to increase the signal-to-noise-ratio of the system, which is the ratio of the magnitude of the signal to the magnitude of the noise present in the system. As unwanted radiation is filtered from the system, a theoretical best noise may be achieved through heterodyne detection. In this regard, heterodyne systems may become very sensitive and much more effective because unwanted noise may be reduced or eliminated.